The present invention relates to a method of controlling NOx gas emission during treatment of metals in nitric acid solutions by the addition of hydrogen peroxide to the solutions.
Nitric acid is finding wide applications in various industries. However, the pickling treatment of metals in nitric acid solutions generally involves the emission of NOx gas detrimental to the environment and human health. In acid-pickling of stainless steels in a mixed acid solution containing nitric acid and hydrofluoric acid, the dissolution of stainless steels results in the formation of nitrous acid in the mixed acid solution. The nitrous acid thus formed is converted to NO and NO2 through various reactions in the solution, and finally evolved as NOx gas from the solution. Scrubbers or other devices have been used to prevent the discharge of NOx gas into the environment. However, the treatment of NOx gas by scrubber, etc. requires additional equipment cost and routine maintenance of apparatus for treating exhaust gas.
U.S. Pat. No. 3,945,865 proposes to control NOx gas emission by the addition of hydrogen peroxide to nitric acid solutions. However, the patent teaches nothing about means for controlling the addition amount of hydrogen peroxide within suitable range. An excess of hydrogen peroxide is readily decomposed in nitric-hydrofluoric acid systems due to metal ions therein to result in unnecessary waste of hydrogen peroxide. Japanese Patent Application Laid-Open No. 55-134694 proposes to control the addition amount of hydrogen peroxide based on the redox potential of nitrous acid solution. However, since the relationship between the redox potential and the nitrous acid concentration is variable, the addition amount of hydrogen peroxide cannot be controlled precisely.
Thus, an object of the present invention is to provide a method of effectively controlling the NOx gas emission from nitric acid solutions, thereby solving the above problems in the prior art.
As a result of extensive study on controlling the addition amount of hydrogen peroxide, the inventors have found that the electrolytic current during potentiostatic electrolysis of nitric acid solutions has a close quantitative relationship to the nitrite ion concentration in the solutions and the evolution amount of NOx gas, and that the addition of hydrogen peroxide is easily controlled by monitoring the electrolysis current, thereby minimizing the addition amount of hydrogen peroxide required for controlling NOx gas emission.
The inventors have further found that the NOx gas emission is effectively controlled by combinedly monitoring the potentiostatic electrolytic current and the redox potential, while avoiding excessive addition of hydrogen peroxide. The present invention has been accomplished based on these findings.
Thus, in a first aspect of the present invention, provided is a method of controlling NOx gas emission from a solution containing at least nitric acid, the addition amount of hydrogen peroxide to the solution is regulated depending on electrolytic current monitored during potentiostatic electrolysis of the solution.